The present invention generally relates to iron-nickel-chromium alloys. More particularly, this invention relates to an iron-nickel-chromium austenitic alloy having a composition that results in the formation of fine (TixZr1-x)(CyN1-y) precipitates in an amount sufficient to play a role in grain refinement and enhance the elevated temperature strength of the alloy.
Various alloys have been considered and used for shrouds, retaining rings, combustor liners, nozzles, and other high-temperature components of turbomachinery, with preferred alloys being chosen on the basis of the particular demands of the application. Shrouds, which surround the outer blade tips within the turbine section of a turbomachine, such as a gas turbine engine, require good low cycle fatigue and oxidation properties.
Many iron-nickel-chromium (Fe—Ni—Cr) austenitic alloys have been developed for turbomachinery, steel and chemical industry components, such as engine valves, heat-treating fixtures and reaction vessels. Fe—Ni—Cr alloys exhibit good oxidation and creep resistances at elevated operating temperatures, such as those within the turbine section of a turbomachine. To promote their elevated temperature properties, Fe—Ni—Cr alloys have been formulated to contain carbide and nitride-forming elements such as niobium and vanadium. Examples of such alloys include those disclosed in U.S. Pat. Nos. 4,853,185 and 4,981,647 to Rothman et al. According to Rothman et al., controlled amounts of nitrogen, niobium (columbium) and carbon are used in a defined relationship to ensure the presence of “free” nitrogen and carbon. Niobium is said to be required in an amount of at least nine times greater than the carbon content. Nitrogen is said to act as an interstitial solid solution strengthener and also form nitrides to provide an additional strengthening mechanism. However, strong nitride formers, such as aluminum and zirconium, are disclosed as being limited to avoid excessive initial coarse nitrides, which are said to reduce strength. Finally, the presence of niobium, vanadium or tantalum in the alloy is said to permit the presence of a very small amount of titanium (not over 0.20 weight percent) for the purpose of providing a beneficial strengthening effect. Rothman et al. teach that higher titanium contents result in the precipitation of undesirable, coarse titanium nitride particles.
Fe—Ni—Cr austenitic alloys of the type described above have found use in shroud applications. However, austenitic alloys are prone to grain growth during forging and heat-treating processes, resulting in reduced low cycle fatigue performance. Most precipitates in these alloys cannot effectively prohibit grain growth during thermomechanical processing because the precipitates are not stable at the required processing temperatures. As a result, a uniform and fine grain structure is often not achieved, especially in the production of large shroud forging rings, to the extent that an unacceptable low cycle fatigue performance results.
In view of the above, it would be desirable if an alloy were available that exhibited desirable properties for forgings intended for high temperature applications, including turbomachinery shrouds and rings.